Filling level sensors known from the prior art are provided with a capacitive sensor element which is arranged in a fluid whose filling level is to be detected. Such known filling level sensors additionally comprise evaluation circuits by means of which a signal indicative of the filling level is produced on the basis of the detected capacitance of the capacitive sensor -element.
These known filling level sensors are disadvantageous insofar as the electrodes of the capacitive sensor elements must be protected. Normally, this means that the electrodes are coated with a protective material. Such protective coatings are often porous so that the fluid whose filling level is to be detected can come into contact with the electrodes of the sensor element, the electrodes consisting e.g. of copper.
Such contact is undesirable especially in cases of use where the filling level of a food fluid, e.g. milk in a respective tank, is to be detected. In this case, the food fluid to be detected may become contaminated or a change in the taste of such foodstuffs may occur.
A further disadvantage of these known filling level sensors is that their capacitive sensor elements, i.e. their electrodes coated with a protective material, have a comparatively complicated structure necessitating an expensive multi-step production.
Still another disadvantage of these known filling level sensors is that part of the fluid whose filling level is to be detected adheres to the protective coatings, whereby measure errors are caused and an incorrect filling level is indicated.
DE 40 25 400 C1 refers to a method and a probe arrangement for dielectric constant-compensated capacitive filling level measurement. This known device comprises a filling level measuring probe as well as a compensation probe which are immersed in a material contained in a receptacle such that they extend parallel to one another and that their longitudinal direction extends parallel to the inner wall of the receptacle. By means of a RF-generator, which is connected to the measurement capacitance, and a measurement current rectifier, the measurement capacitance of the filling level measuring probe and of the compensation probe, respectively, is determined and the filling level is derived therefrom.
DE 39 02 107 A1 refers to a capacitive level gauge comprising a measuring capacitor consisting of at least one measuring electrode and at least one earth electrode implemented as condutive wires, stranded conductors or ribbons, which extend in parallel, juxtaposed relationship and which are separated from one another and from the medium to be measured by means of a dielectric insulation. In FIGS. 2 to 7 various embodiments of the measuring capacitors are shown.
WO 92/18856 refers to a switched capacitor circuit and deals with the measurement of the electric conductivity of liquids.
SUMMARY OF THE INVENTION
It is the object of the present invention to further develop a filling level sensor in such a way that it can be produced more easily and at a lower price with sufficient protection of the electrodes of the capacitive sensor element of the filling level sensor, and in such a way that measurement errors are avoided when the filling level sensor is in operation.
The present invention provides a capacitive filling level sensor having a first capacitive sensor element arranged close to a fluid whose filling level is to be detected, a second capacitive sensor element arranged close to a fluid whose filling level is to be detected, and a switched-capacitor measuring circuit which is connected to the first and second capacitive sensor elements and which detects the capacitance values thereof. The switched-capacitor measuring circuit produces in response to the detected capacitance values a signal indicative of the filling level. The first capacitive sensor element and the second capacitive sensor element are each defined by two wires of a ribbon cable having at least five wires, the fifth wire extending between the wires which defined the capacitive sensor elements, and being connected to ground.
Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings.